Thanks to remarkable development in the field of biomedical measurement technique, precision in measuring weak electric field (brain wave) or weak magnetic field (brain magnetic wave) generated from the brain, of which measurement has been difficult and error-prone, has been improved year by year.
Specifically, receiving external stimuli, neural cells in the brain generate a current. The current results in the afore-mentioned weak electric field or weak magnetic field. Here, “brain wave” refers to the electric field generated from the brain by the current from the neural cells. An “electroencephalogram: EEG” refers to a method of measuring the brain wave.
The “brain magnetic wave” refers to a magnetic field generated from the brain by the current from the neural cells. A “magnetoencephalography: MEG” refers to a method of measuring the brain magnetic wave. A crucial advantage of the magnetoencephalography is that, as the magnetic field is almost free of any influence of volume conductor, it is expected that relatively accurate three-dimensional estimation of the position of a brain current source can be attained by measuring magnetism from outside one's scalp.
In the analysis of the brain magnetic wave, an active portion of the brain is estimated in a non-invasive manner, by measuring the generated magnetic field from outside the brain.
The magnetic field, however, is so weak that it is very susceptible to the influence of external magnetic field such as terrestrial magnetism. Therefore, the weak magnetic field is measured by a Superconducting QUantum Interference Device (SQUID), which is a measuring device utilizing superconductivity, within a shield that shuts out any external magnetic field.
It is noted, however, that in the field of studying algorithms for “estimating positions of brain current source,” decisive method is non-existent at present, though various and many variations of initial models have been tried.
By way of example, “dipole estimation method” as one algorithm for “estimating positions of brain current sources,” is disclosed in Reference 1: J. C. Mosher, P. S. Lewis and R. M. Leahy, IEEE Trans. Biomed. Engng. <1992> vol. 39, pp.541-557. In the “dipole estimation method,” however, the position of a dipole is estimated from observed magnetic field, assuming that the current source in the brain can be represented by one or a number of current dipoles, and this method is disadvantageous in that it is difficult to determine the number of dipoles.
As another algorithm, “spatial filtering method” is disclosed in Reference 2: K. Toyama, K. Yoshikawa, Y. Yoshida, Y. Kondo, S. Tomita, Y. Takanashi, Y. Ejima and S. Yoshizawa, Neuroscience, 1999, 91 (2), pp. 405-415. In the “spatial filtering method,” location of a brain current source is restricted in consideration of physiological findings, and distribution of dipoles is estimated. This method is disadvantageous in that accurate estimation of the depth of the current source is not possible.